This invention relates to thyristors and to methods of manufacturing thyristors. The thyristor may have a bi-directional conducting capability as well as a bi-directional blocking capability (i.e. it may be a triac), or it may merely have a uni-directional conducting capability and a bi-directional blocking capability.
Thyristors are well-known bipolar semiconductor devices, comprising a semiconductor body having an N-P-N-P region structure between opposite, first and second major surfaces. The N-P-N-P region structure includes a forward base region of a first conductivity type (e.g. p-type) that forms a forward blocking p-n junction with a higher-resistivity base region of the opposite second conductivity type. The base region of the second conductivity type is herein termed the xe2x80x9creversexe2x80x9d base region. It is mainly in this high-resistivity region that high voltage is carried across a depletion layer in both the forward and reverse blocking states of the thyristor. The reverse base region separates the forward blocking junction from a reverse blocking junction formed with an underlying region of the first conductivity type. In the forward blocking state, the voltage-carrying depletion layer extends in the high-resistivity reverse base region from the forward blocking junction. In the reverse blocking state, the voltage-carrying depletion layer extends in the high-resistivity reverse base region from the reverse blocking junction.
European patent specification EP-B-0 519 268 describes a bipolar N-P-N planar transistor having a p-type base region that forms a reverse-biased p-n junction with a higher-resistivity collector region. The transistor is designed for high temperature operation (i.e. above 200xc2x0 C.). For this reason, a lower-doped p-type perimeter zone is provided to form the perimeter of the base region and terminates the p-n junction at a first major surface. The perimeter zone extends from the first major surface to a greater depth in the collector region than the base region. The perimeter zone acts as a guard ring that reduces the surface peak field at the outer perimeter of the base region so as to provide leakage current stability at temperatures above 200xc2x0 C. It is stated in col.5 line 10 that the invention of EP-B-0 519 268 can also be used for thyristors and diodes. The whole contents of EP-B-0 519 268 are hereby incorporated herein as reference material.
The following further statements are made in col.2 line 54 to col.3 line 16 of EP-B-0 519 268:
that the resulting thermal stability at temperatures above 200xc2x0 C. is achieved independent of any surface charges present in any glass passivation layer;
that the magnitude of reverse voltage that can be blocked by the p-n junction is dependent on the dimensioning and doping of the base region and of its perimeter zone;
that a further increase in the thermally stable reverse voltage can be achieved when an even deeper, mesa trench adjoins directly the perimeter zone and surrounds the latter in annular form. FIG. 2 of EP-B-0 519 268 illustrates an N-P-N transistor with such a mesa trench, whereas FIGS. 3 and 4 of EP-B-0 519 268 illustrate N-P-N transistors without mesa trenches.
It is an aim of the present invention to provide a thyristor having a blocking voltage capability that is substantially independent of the layout geometry of the active thyristor area, and thereby to facilitate its design and its manufacture.
According to a first aspect of the present invention, a thyristor having a forward base region with a lower-doped perimeter zone is characterised by a concentric arrangement of the perimeter zone of the forward blocking p-n junction with an outer perimeter zone of the same first conductivity type that brings the reverse blocking p-n junction of the thyristor to the first major surface at a lateral distance around the forward blocking p-n junction. The outer perimeter zone has a doping profile and a depth that are the same as a doping profile and the depth of the lower-doped perimeter zone of the forward base region. The outer perimeter zone extends in depth to a lower perimeter zone of the underlying region of the first conductivity type which forms the reverse blocking junction with the other base region (reverse base region) that is of the opposite (second) conductivity type.
All these perimeter zones together provide the thyristor with a deep peripheral termination which surrounds an active thyristor area that conducts in an on-state of the thyristor. The low-doped inner and outer perimeter zones are much deeper than the surrounded forward base region, typically at least 35 xcexcm deeper and/or at least twice the depth of the forward base region. By making these perimeter zones so much deeper, the forward and reverse the blocking voltage capability of the thyristor is determined substantially independently of the layout geometry of the active thyristor area.
Whereas the low-doped perimeter zone of EP-B-0 519 268 was adopted to provide high temperature devices (above 200xc2x0 C.), the concentric perimeter zone arrangement of the present invention is adopted to effectively de-couple the design of the active thyristor area from the determination of the blocking voltage capability. Thus, most thyristors with perimeter zones in accordance with the present invention may be designed for operation at more normal temperatures, e.g. in a range from about 125xc2x0 C. to 150xc2x0 C. Furthermore, the inner and outer perimeter zones can terminate at a plane surface area of the top major surface, at a passivating layer thereon, without needing to provide any deeper mesa trench inbetween.
The doping of the forward base region and layout geometry of the N-P-N-P region structure in the active thyristor area can be chosen to give different magnitudes of transistor gain and different thyristor switching characteristics, while maintaining a given blocking voltage capability by means of the concentric perimeter zone arrangement of the peripheral terminations of the forward and reverse blocking p-n junctions. Furthermore, most thyristors comprise emitter-base shorts in the active area, which result in a pattern of current paths in the base region. It is necessary to lay out the pattern of emitter-base shorts of prior-art planar thyristors in such a way that these current paths are not adversely influenced by the highly doped (and hence highly conductive) perimeter of the base region. Current conduction in the lower doped perimeter zone of thyristors in accordance with the present invention is less and may even be pinched off from the active area, so easing the design of emitter-base shorts in the active area.
With a given peripheral termination, thyristors of the same type but a wide range of differing on-characteristics may be formed by varying the active thyristor area. Alternatively, different active thyristor areas may be incorporated to give different thyristor types having a given blocking voltage capability as determined by their concentric perimeter zone arrangement.
Most types of thyristor comprise a gate electrode coupled to the forward base region. Different types of gated thyristor may be designed with a concentric perimeter zone arrangement terminating their forward and reverse blocking p-n junctions in accordance with the present invention. The gate electrode may ohmically contact the forward base region in a simple thyristor or in an amplifying-gate thyristor. It may be capacitively coupled to the forward base region via a dielectric layer in, for example, a MOS-gated thyristor. Generally the gate electrode serves to turn on the thyristor from a blocking state. When the thyristor has an ohmic gate electrode, it may be additionally designed to also turn off the thyristor by diverting current from the base region into the gate circuit.
However, instead of having a gate electrode, the thyristor may be of the optically-triggered type. In this case, incident light (from, for example, a light-guide or light-emitting diode or laser in the thyristor device package) turns on the thyristor by generating electron-hole pairs in the base region. An optically-triggered thyristor may be designed with a concentric perimeter zone arrangement terminating its forward and reverse blocking p-n junctions in accordance with the present invention. The thyristor may be a triac, and so it may have a bi-directional conducting capability as well as a bi-directional blocking capability. Such a device is effectively a monolithic integration of two thyristors in an anti-parallel configuration. In this case, the underlying region of the first conductivity type acts as an emitter region of the forward-conduction thyristor and as a base region of the reverse-conducting thyristor. As explained below, the edge termination scheme in accordance with the present invention may be additionally arranged to have the top emitter region extended into the low-doped perimeter zone of the forward base region, so as to overcome a problem present in prior-art planar triacs. However, the thyristor may merely have a uni-directional conducting capability and a bi-directional blocking capability. In this case, the underlying region of the first conductivity type acts only as an emitter.
The lateral distance (i.e. spacing) between the forward and reverse blocking p-n junctions is determined by their concentric perimeter zones. Preferably, this lateral distance/spacing is no smaller than (and is preferably larger than) the smallest vertical thickness of the high-resistivity base region between the forward and reverse blocking p-n junctions in the active thyristor area. This spacing arrangement is advantageous in avoiding pre-mature breakdown that might otherwise result from reach-through of the depletion layer between the forward and reverse blocking p-n junctions at the first major surface. It is preferable for breakdown to occur in the bulk of the device body, rather than at the surface. It is also preferable for the vertical thickness of the high-resistivity base region to be sufficiently large to avoid punch-through of the depletion layer at the maximum applied voltage. Thus, when excessive voltages are present, it is generally preferable for an avalanche breakdown of the blocking p-n junction to take place before punch-through of the depletion layer can occur.
The outer perimeter zone comprises a same doping profile as the perimeter zone of the forward base region. It also has the same depth as the perimeter zone of the forward base region. These perimeter zones can be easily provided in the same doping step or steps. Their mutual spacing can be determined with good reproducibility by the spacing of windows in a masking pattern. The perimeter zones may also comprise additional shallower doping profiles.
According to another aspect of the present invention, there are provided advantageous methods of manufacturing thyristors with a concentric perimeter zone arrangement terminating their forward and reverse blocking p-n junctions in accordance with the invention.
In one such advantageous method, with a semiconductor body of silicon, the concentric arrangement of the inner perimeter zone (of the forward base region) and the outer perimeter zone may be formed by steps that include the implantation and diffusion of aluminium ions at the first major surface of the body. The lower perimeter zone may have a doping profile formed by diffusion from a metal aluminium pattern at the second major surface of the silicon body. The resulting lower perimeter zone may extend through more than half the thickness of the body. Windows in a masking pattern at the first and second major surfaces can be used to localise these doping processes. The diffusion of all these perimeter zones may be carried out in one or more common heating steps.
In order to optimise doping profiles and/or junction curvatures, the perimeter zones of a thyristor in accordance with the invention may comprise a shallow portion having a different dopant from a deep portion. Thus, for example, the shallow portion may be doped with boron and the deep portion may be doped with aluminium. For the perimeter zone of the forward base region it is usually preferable that both the shallow and deep portions are more lowly doped than the forward base region. Furthermore in this case, the shallow portion may extend laterally beyond the deep portion to extend the termination of the forward blocking p-n junction at the first major surface.